1. Field of the Invention
The present invention relates to an upshift control device for an automatic transmission, and more particularly to a technique for controlling a back pressure of an accumulator for an engaging element to be released in the course of upshifting.
2. Description of the Prior Art
There is known a shift control device for an automatic transmission as disclosed in Japanese laid-open Patent Publication No. 5-248526. As shown in FIG. 12 (and FIG. 5 of this publication), when the transmission is shifted up from its 3rd-speed gear position to 4th-speed gear position by releasing a low clutch 50 and engaging a band brake 28 (not shown in FIG. 12), a low clutch sequence valve 90 is switched from a supply position in which a back pressure is supplied to a low clutch accumulator 83 of the low clutch 50, to a drain position in which the accumulator is drained, when a difference .DELTA. P between an accumulator back pressure and a 4th-speed engaging pressure as operating signal pressures becomes equal to a predetermined value (for example, .DELTA.P=0.5 kg/cm2).
In this arrangement, when the transmission is shifted up from the 3rd-speed to 4th-speed gear position while the accelerator pedal is being depressed and the back pressure of the accumulator is at a relatively high level, the low sequence valve 90 is switched to the drain position immediately after a servo piston stroke of the band brake 28 is completed, so that the low clutch pressure is reduced and the 4th-speed engaging pressure is increased in optimum timing.
If the 3-4 upshifting occurs when the accumulator back pressure is at a low level in the conventional shift control device, however, the low clutch sequence valve 90 is switched to the drain position in early timing before completion of the servo piston stroke of the band brake 28, as shown in FIG. 14, to thus bring both the low clutch 50 and band brake 28 into released positions, resulting in racing of the engine.